A kryptonite bike lock and a chain to go through your rear tire. No one is going to want to mess with it.

It's about 5km, and a fairly nice ride despite my city doing very little for cyclists. No theft so far! I see there's been lots of discussion on theft prevention but I'll add my opinion: you need to make your bike less of a target by having a big scary looking lock on a bike that doesn't look great. Just oversize the lock to such a degree that it's not worth messing with. Also locking up in a higher traffic, more visible area is helpful.